Pulse 4 peanut butter cups in a food processor into small chunks; transfer to a bowl. Pulse the remaining 6 peanut butter cups into a fine powder; transfer to another bowl. Chill both until ready to use.
Preheat the oven to 350 degrees F. Grease four 8-inch round cake pans with cooking spray. Dust flour and knock out any excess. Set aside.
In a medium bowl, sift together the flour, baking powder and salt.
Cut up the butter into 1-inch pieces and place them in the large bowl of an electric mixer fitted with a paddle attachment or beaters. Beat on medium-high speed until the butter is light and creamy in color, about 3 minutes. Stop and scrape the bowl. Cream the butter for an additional minute. Add the granulated sugar, 1/4 cup at a time, beating 1 minute after each addition. Scrape the sides of the bowl occasionally. Add the eggs one at a time. Reduce the mixer speed. Stir the vanilla into the buttermilk. Add the dry ingredients to the butter alternately with the buttermilk. Mix just until incorporated. Scrape the sides of the bowl and mix for 15 seconds longer.
Divide half the batter between the prepared cake pans. Set the remaining batter aside. Using the back of a spoon, spread out the batter so that it covers the bottom of each pan, making it slightly thicker around the edges than in the middle. Bake until cooked through and golden around the edges, 12 to 14 minutes. Set aside to let cool slightly, then loosen the cake layers with a knife and invert onto cooling racks.
Wash and dry the cake pans. Repeat the process a second time with the cooking spray, flour and batter.
Once all 8 layers have baked, place the first layer on a serving plate. Spread with about 3 tablespoons Chocolate Frosting and sprinkle with about 1 tablespoon powdered peanut butter cups. Top with another cake layer and repeat the process to make 8 layers in all. Frost the outside of the cake with the remaining frosting and sprinkle the top with the peanut butter cup chunks.
In a large saucepot over medium-high heat, bring the milk to a simmer, and then remove from the heat.
In a large mixing bowl, whisk together the yolks, flour, 1/3 cup confectioners' sugar and a pinch of salt. Add the hot milk in a stream, whisking. Return the mixture to the saucepot and bring to a boil over moderate heat. Whisk until very thick, about 3 minutes. Transfer to a large bowl. Cover the surface of the custard with a buttered round of wax paper and cool completely, about 45 minutes.
Using electric beater, add the vanilla and remaining 1 cup confectioners' sugar to the custard. Beat at moderate speed until well combined. Then increase the speed to medium-high and beat in the butter, 2 tablespoons at a time, until smooth. Add the chocolates and beat until well combined.